1. Field of the Invention
The present invention relates to a transmission-reflection double type liquid crystal display device which can be reduced in thickness and weight easily and which is excellent in display quality.
The present application is based on Japanese Patent Application No. Hei. 11-369712, which is incorporated herein by reference.
2. Description of the Related Art
A liquid-crystal display device in which a back-lighting system using a bottom-lighting type or side-lighting type light pipe is disposed on a back side (opposite to a visual side) of a transmission type liquid-crystal display panel through a half-transmission type reflector is heretofore known as a reflection-transmission double type liquid-crystal display device which can be viewed in a transmission mode by using a built-in illuminator in addition to a reflection mode by using external light. The half-transmission type reflector is disposed in order to enable viewing in a reflection mode. If there is no half-transmission type reflector, viewing in a reflection mode by using external light is so dark that the liquid-crystal display device substantially hardly functions as a reflection type liquid-crystal display device.
The background-art reflection-transmission double type liquid-crystal display device using a back-lighting system, however, has the following problem. That is, it is difficult to reduce the thickness, size and weight of the liquid-crystal display device though greater reduction in thickness, size and weight of the liquid-crystal display device has been demanded for the purpose of reducing in size and weight of a portable telephone set, a portable personal computer, or the like. Incidentally, in a bottom-lighting type back-lighting system, a thickness of not smaller than 4 mm is generally required for a light diffusing plate and a reflector which are disposed together with the illuminator. In a side-lighting type light pipe, a plate thickness of not smaller than 1 mm is required for light transmission. If a light diffusing plate, a reflector, a prism sheet, etc. are further disposed on the side-lighting type light pipe, a further thickness of not smaller than 3 mm is generally required. If at least one half-transmission type reflector is still further added, the volume and weight of the liquid-crystal display device become even larger. Moreover, there is a problem that viewing in a transmission mode becomes dark because of the arrangement of the half-transmission type reflector and that brightness in a reflection mode is inferior to that of a reflection exclusive type liquid-crystal display device using a high-reflectance reflection layer.
An object of the present invention is to develop a transmission-reflection double type liquid-crystal display device which can be reduced in thickness and weight easily and which is excellent in display quality.
According to the present invention, there is provided a reflection-transmission double type liquid-crystal display device comprising: a transmission type liquid-crystal display panel including a liquid-crystal cell; at least one illuminator disposed on at least one of side surfaces of the liquid-crystal display panel and capable of being switched on/off; an optical path changing sheet which has a refractive index exhibiting a refractive index difference of not higher than 0.15 from a refractive index of a nearest liquid-crystal cell substrate, and which is bonded onto a back side (opposite to a visual side) of the liquid-crystal display panel through an adhesive layer having a refractive index exhibiting a refractive index difference of not higher than 0.20 from the refractive index of the nearest liquid-crystal cell substrate; and a reflection layer disposed on a back side of the optical path changing sheet; the optical path changing sheet including optical path changing slopes and flat surfaces, each of the optical path changing slopes facing the illuminator at an inclination angle in a range of from 30 to 48 degrees with respect to a plane of the optical path changing sheet and being provided for reflecting incident light from the illuminator toward the visual side of the liquid-crystal display panel, each of the flat surfaces being inclined at an inclination angle of not larger than 10 degrees with respect to the sheet plane so that a projected area of the flat surfaces on the sheet plane is not smaller than 10 times as large as a projected area of the optical path changing slopes on the sheet plane.
According to the present invention, while incident light from an illuminator disposed on one of side surfaces of a liquid-crystal display panel is transmitted backward efficiently by use of liquid-crystal cell substrates, the optical path of the transmission light is changed efficiently toward the visual side of the liquid-crystal display panel through an optical path changing sheet disposed on the back side of the panel. Hence, the transmission light can be utilized for liquid-crystal display in a transmission mode. Moreover, external light can be transmitted/reflected efficiently through/by flat surfaces of the optical path changing sheet and a reflection layer. Hence, the external light can be utilized for liquid-crystal display in a reflection mode. The illuminator which is disposed on the side surface, the optical path changing sheet which is excellent in thickness, and the reflection layer can form a back-lighting (transmission mode) system and a reflection mode system. Hence, a transmission-reflection double type liquid-crystal display device which is excellent in thickness and light in weight, and which is bright and excellent in display quality can be formed.
The aforementioned effect is based mainly on use of a slope reflection type optical path changing sheet. That is, light incident on a side surface or transmission light of the incident light is reflected by slopes of the optical path changing sheet so that the optical path of the light can be changed with good directivity. Hence, good visibility in a transmission mode can be achieved. Moreover, external light is transmitted through flat surfaces of the optical path changing sheet so that the external light can be kept sufficiently. Hence, good visibility in a reflection mode can be also achieved. In a method of scatter reflection by a roughened surface, it is difficult to achieve the aforementioned effect. Incidentally, JP-A-5-158033 discloses a reflection type liquid-crystal display device in which illumination light is made incident on one of side surfaces of a liquid-crystal display panel and totally reflected by a visual side cell substrate and in which the reflected light is scattered by a rough surface type reflector so that the scattered light is utilized for display.
In the aforementioned case, however, light allowed to be utilized for display is light that comes out from the panel due to coming contrary against the total reflection condition by scattering. Generally, scattered light exhibits a normal distribution having a direction of regular reflection as a peak, in Extended Abstracts (20th Liquid-Crystal Discussion Lecture Vol. 3 G510, Tohoku University; Uchida et al). Hence, the aforementioned display light is light largely inclined with respect to a frontal (vertical) direction and therefore hardly utilized effectively for display. Hence, the display becomes dark in the frontal direction. Nevertheless, intensifying scattering by the roughened surface type reflector is unfavorable to display in a reflection mode because the quantity of light in the frontal direction in the reflection mode is reduced (SID 96 DIGEST pp.149-152). It is, therefore, necessary to adjust scattering intensity to keep balance between both transmission and reflection modes in such a roughened surface type scattering reflection method. It is, however, difficult to obtain scattering intensity favorable to the two reflection and transmission modes because scattering intensity required in the transmission mode is antinomic to scattering intensity required in the reflection mode.
On the other hand, the slope reflection type optical path changing sheet according to the present invention mainly utilizes light exhibiting a peak in a direction of regular reflection and controls the optical path of the reflected light. Hence, directivity, especially frontal directivity, favorable to display can be provided easily, and a bright transmission mode can be achieved. Also in a reflection mode, flat portions of the optical path changing sheet except the slopes can be utilized, and efficient entrance, reflection and transmission of external light can be ensured. Hence, the state of light can be balanced easily so as to be favorable to both transmission and reflection modes.
Features and advantages of the invention will become understood from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.